The goals of the proposed investigation are to study the regulation of the tyrosine phosphatase STEP. Previous studies established that STEP inactivates ERK1/2, p38 and the tyrosine kinase Fyn. Over the last cycle, we have discovered that STEP also induces NR1/NR2B and GluR1/GluR2 internalization. STEP has been implicated in the pathophysiology of Fragile X syndrome (FXS), Alzheimer's disease (AD), and schizophrenia (SZ). In these disorders, there is an increase in STEP levels. The current model suggests that the increase in STEP leads to the inappropriate internalization of NR1/NR2B and GluR1/GluR2 receptors. However, the mechanisms by which STEP levels are increased differ in these disorders. In AD and SZ, there is a disruption in the ubiquitination and degradation STEP. In FXS, there is an increase in the translation of STEP mRNA due to the absence of FMRP. STEP is rapidly degraded after specific forms of synaptic activity. Synaptic stimulation results in the ubiquitination of STEP, which is then degraded through the proteasome system. We hypothesize that STEP is degraded at synaptic sites to promote synaptic strengthening. Extrasynaptic NMDAR stimulation results in a calpain-mediated proteolysis STEP. We propose that this mechanism prolongs p38 signaling and initiates cell death pathways. The molecular mechanisms that regulate these processes are unknown. Aim 1 focuses on STEP ubiquitination. We include functional experiments that mutate STEP so that it can no longer be ubiquitinated and degraded. We predict that this construct will disrupt both LTP, as well as the consolidation of fear conditioning after infusion into the amygdala. We will determine the kinases that promote or prevent STEP ubiquitination. Aim 2 focuses on STEP proteolysis. We include functional studies that disrupt proteolysis of STEP, and will determine the effects this has on cell death using in vivo excitotoxic models. We predict that blocking STEP proteolysis will significantly decrease cell death. Aim 3 focuses on STEP translation. Two cytoplasmic polyadenylation elements (CPEs) are present in STEP mRNA and we will determine whether these elements regulate the translation of STEP mRNA. We include functional experiments that will mutate these CPEs. We predict this will disrupt the normal function of STEP in the regulation of its substrates, including glutamate receptor trafficking. These studies will advance our understanding of how STEP is involved in normal cognitive processes and possible mechanisms by which it is disrupted in several diseases in which there are significant cognitive deficits.